"If the under side of the mantle be left broad and flat... the cloud of dust or light ashes, that rises from a coal fire nearly burnt out, when... violently stirred... with a poker, striking perpendicularly against this flat part... must... be beat back into the room; but when the breast of the chimney is properly rounded off, the ascending cloud of dust and smoke more easily finds its way into the throat of the chimney, and is even directed and assisted in some measure by the warm air of the room that gets under the mantle and is going the same way."